The present invention relates to a power supply system having a plurality of power supply devices and an electronic device including such a power supply system.
A power supply device such as a primary battery including a dry battery, a secondary battery such as a lithium ion battery, and a power generating device such as a fuel cell and a solar cell is used as an assembled battery in which a plurality of cells are connected in series or in parallel with each other to meet electricity demand of electronic devices.
However, in the case where characteristics and deterioration degree of each power supply device varies, a specific power supply device suffers overload. Accordingly, characteristics as an assembled battery are lowered. In some cases, a dangerous failure is caused thereby.
For example, in the case where an assembled battery is configured by connecting a plurality of cells in series, variation of the respective cells becomes an enormous problem. Specifically, in the case where each internal resistance of each cell varies, heat value is increased in the cell having high internal resistance, and deterioration in the cell rapidly proceeds particularly. If deterioration proceeds, the internal resistance is more increased. Finally, voltage drop due to internal resistance exceeds electromotive force, resulting in “polarity inversion state” in which inter-terminal voltage is reversed. Further, in such a polarity inversion state, output as an assembled battery is significantly lowered (For example, refer to Patent documents 1 and 2).
Meanwhile, in the case where an assembled battery is configured by connecting a plurality of cells in parallel, variation of the respective cells becomes an enormous problem as well. In the case where a plurality of each cells having different electromotive force are connected in parallel, a current is flown from the cell having high electromotive force to the cell having low electromotive force. Such backflow phenomenon more easily occurs as the internal resistance of the cell is lower in principle. Thus, as the cell performance is improved more and the internal resistance is decreased more, backflow current is more increased, which is so-called trade-off.
Here, in the case where an assembled battery is configured by connecting power supply devices such as a dry battery, a fuel cell, and a solar cell not available for charge (battery backflow) in series, variation of the respective cells becomes more enormous problem. For example, in the case of the dry battery, if a new dry battery and an old dry battery are connected in parallel, a current is flown from the new battery having high electromotive force to the old battery, and thus air bubbles and the like are generated in the old battery, possibly resulting in rupture. Further, in the case of the fuel cell and the solar cell, a current is forcefully flown from a cell having high electromotive force to a cell having low electromotive force, and thus deterioration of electrode materials may be promoted (for example, refer to Patent document 3).
Further, in the case where an assembled battery is configured by connecting active fuel cells in which a fuel is supplied by a pump or the like in parallel, the state thereof is more serious. In the active fuel cell, the fuel supply amount easily becomes pulsative. That is, time change of electromotive force is significantly large. Therefore, in order to obtain stable power generation of a plurality of fuel cells, it is desirable to independently control fuel spouting. However, due to such independent control, each electromotive force varies among the plurality of fuel cells.
Further, in the case where an assembled battery is configured by connecting solar cells in parallel, the state thereof is more serious similarly. This is because in the solar cell, if part thereof is shaded, its electromotive force is changed.